The present invention relates to a Hall sensing apparatus useful for generating a current signal having a fixed phase relationship to the current in a power distribution conductor and, more particularly, to a Hall sensing apparatus having a large dynamic range, offset dc bias control and temperature drift control.
Hall effect sensor devices which take advantage of the Hall effect are well known. In such devices, Hall current in a Hall element is deflected by a magnetic field at right angles to both the direction of Hall current and the direction of the applied magnetic field. The Hall current deflection results in a Hall output voltage across the Hall element which is proportional to the vector cross product of the Hall current and the perpendicular magnetic flux.
Numerous devices, such as field detectors, modulators and various transducers, incorporate Hall sensor elements. The present invention incorporates a Hall sensor element to detect the phase of a current in a power distribution system conductor. Such a device is useful in the phase angle meter instrument disclosed in my co-pending application Ser. No. 042,671 filed on July 2, 1979, in which a current signal having a fixed phase relationship with the conductor current is combined with a voltage signal having a fixed phase relationship with the voltage between a pair of power distribution conductors, to allow the phase angle and power factor to be readily determined.
In general, the potential gradient across the Hall element is porportional to the applied magnetic field and the Hall current. However, when the Hall element is used to generate a signal in phase with a distribution system conductor current of only several amps, it is difficult to extract the potential gradient signal from the inherently large noise on the distribution conductor. Furthermore, when the distribution system conductor current is relatively large, the potential gradient across the Hall device can exceed the power supply voltage of the signal processing circuitry resulting in serious inaccuracies and even circuit component damage. Hence, it is desired to increase the signal-to-noise ratio of the potential gradient signal when relatively low conductor currents exist and to limit the potential gradient signal when relatively high conductor current exist to thereby provide a Hall sensor device with a large dynamic range (about 5 to 800 amps).
Another major drawback of utilizing a Hall sensor element in the aforementioned application is that a dc offset voltage will generally be present. This dc offset significantly effects the determination of a positive and negative going zero voltages crossings of the potential gradient signal and, hence, the generation of a current signal in phase with the distribution conductor current. Hence, it is desired to provide for detection and compensation of any dc offset voltage which may exist.
Finally, it is well known that Hall effect devices are generally sensitive to temperature resulting in temperature drift. This adversely affects the relative phase of the potential gradient signal. In order to minimize this problem, it is desired to minimize the heating of the Hall sensor element due to the flow of Hall current.
The present invention provides a large dynamic range by incorporating a novel magnetic field concentrator assembly having two magnetic field pick-up plates for being positioned on either side and adjacent to the distribution system conductor. A concentrator rod is attached to a center region of each pick-up plate in a generally symmetrical arrangement about the periphery of the conductor 12. The ends of the concentrator rods remote from the pick-up plates are juxtaposed opposite one another with a Hall sensor element positioned in a space therebetween. A magnetic field is thus impressed across the surface of the Hall sensor element between the juxtaposed surfaces of the concentrator rods.
When there is a relatively low current, e.g., about 5 amps, in the distribution conductor, the pick-up plates operate to pick-up additional magnetic field flux and concentrate that flux in the concentrator rods. Since the inherent noise of the distribution system is generally independent of the magnetic field impressed across the Hall sensor element, the pick-up plates result in an increase in the magnitude of the potential gradient signal of the Hall sensor element without affecting the noise. Hence, the signal-to-noise ratio is increased permitting signal amplification and filtering utilizing conventional components.
As previously indicated, a relatively high current on the distribution conductor produces an output voltage signal which is frequently higher than the voltage of the power supplied to the electrically coupled amplifier and filter circuitry. However, it is known that there is an upper limit to the amount of magnetic flux which can be concentrated in a given area. Hence, the magnetic field flux across the Hall sensor element between the ends of the concentrator rods can be limited by limiting the cross-sectional area of the concentrator rods.
In the preferred embodiment, concentrator rods having a diameter of 3/16 inches were found to saturate and, hence, limit the magnitude of potential gradient across the Hall sensor element to a value less than the power supply voltage of the amplifier and filter circuitry.
In order to provide a signal with a zero volt dc bias, a dual differential amplifier network is coupled across the Hall sensor voltage outputs to generate an inverted and a non-inverted signal. These two signals are summed and compared against a reference voltage, such as ground, in a summing amplifier to generate a dc output signal which is coupled to control the amount of Hall current in the Hall sensor element. The resistors in the Hall current circuit, including the resistance of the Hall device itself, form a voltage divider. A summing amplifier controls the magnitude of the Hall current and, by using feedback control, adjusts the voltage divider such that a zero voltage bias appears on the Hall voltage terminals of the Hall voltage sensor device.
Finally, in order to minimize temperature drift in accordance with the present invention, a second current control device is provided in the Hall current circuit so that Hall current exists in the Hall element only at preselected periods of time. The current control device is activated and deactivated in response to command signals. This limits errors due to temperature drift by limiting the time during which Hall current flows in the Hall sensor element.